The invention relates to an electromagnet which can be applied in a particularly advantageous manner to a proportional magnet, which is arranged within a hydraulic system of an apparatus for varying the control times of inlet or outlet valves for an internal combustion engine, for operating a hydraulic valve.
DE 195 04 185 A1 discloses an electromagnet of this general type for operating a hydraulic valve. It has a coil former which is fitted with at least one coil winding and has an external circumference surrounded by a magnet housing. At the end, this coil former is bounded by an upper pole shoe, which is formed by an annular pole disk with a pole tube inserted in it and on which an electrical connecting body rests. It is also bounded by a lower pole shoe, which is formed by a pole plate with an integrally formed pole core and projects into the hollow cylinder of the coil former. The hollow cylinder of the coil former is clad with a nonmagnetic metal tube, having a cavity in the form of an armature space for a cylindrical magnet armature which moves axially. The magnet armature in turn divides the armature space into a first chamber and a second chamber, which are connected to one another via a number of eccentric axial holes in the magnet armature, in order to equalize the pressure of operating fluid which enters the armature space via the hydraulic valve. Furthermore, a push rod is mounted in a central basic hole in the valve-side end face of the magnet armature, is passed through a likewise central axial hole in the lower pole shoe, and is connected to a control piston which is arranged in the interior of a valve housing of a hydraulic valve. The valve housing of the hydraulic valve case rests on the lower pole shoe of the electromagnet, forming a seal. The interior of the valve housing, which guides the control piston, is connected to the first chamber of the armature space via a further eccentric hole, which is arranged alongside the central axial hole, in the lower pole shoe for pressure equalization.
However, this known electromagnet has the disadvantage that its individual parts require precise and costly manufacture and a high level of installation complexity due to their design configuration and their arrangement with respect to one another, causing production of such an electromagnet to be expected to be very costly. For production engineering, for example, it has been found to be very costly to design the magnet armature and the push rod as an assembly in which these items are firmly connected to one another, while at the same time passing the push rod through the central axial hole in the lower pole shoe. This requires complex calibration work on all the parts to avoid axial offsets between the longitudinal axis of the magnet armature and the longitudinal axis of the push rod, and between the push rod and the longitudinal axis of the central axial hole in the lower pole shoe. Such axis offsets would cause the radial air gaps between the magnet armature and the armature guide and/or between the push rod and the axial hole to not be of equal magnitude. In consequence, the magnet armature or the push rod thus rest on the armature guide or on the axial hole at one point when a current or flow is passed through the electromagnet, causing a friction force to act on the magnet armature in the opposite sense to its movement direction. This could lead to unacceptably high hysteresis. Furthermore, however, the eccentric pressure equalizing channels arranged in the magnet armature and in the lower pole shoe have been found to be highly costly, since they normally have to be drilled, and eccentric incorporation of these holes significantly increases the manufacturing costs.
The invention therefore has the object of providing an electromagnet, in particular a proportional magnet for operating a hydraulic valve, wherein its individual parts and their arrangement with respect to one another are physically simple, involve a low level of manufacturing and assembly effort, and have optimized-cost production. At the same time, it optimally guides the magnet armature and the push rod and has adequate capabilities for pressure equalization between the first chamber and the second chamber of the armature space, as well as between the first chamber and the interior of a valve housing.
According to the invention, this object is achieved for an electromagnet wherein the push rod, which is guided in the axial hole in the lower pole shoe, is in the form of a loose profiled rod which is separated from the magnet armature. The cross-sectional shape of the rod is different from that of the axial hole and its cross-sectional area is less than that of the axial hole, so that the free cross-sectional spaces within the axial hole in the lower pole shoe may also be used as pressure equalizing channels between the interior of the valve housing of the hydraulic valve and the first chamber in the armature space of the electromagnet. The separation of the push rod from the magnet armature of the electromagnet has the advantages that it is no longer possible for any axis offsets to occur between the longitudinal axis of the magnet armature and the longitudinal axis of the push rod, or between the latter and the longitudinal axis of the axial hole in the lower pole shoe, and that both the magnet armature and the push rod can thus be guided optimally, separately from one another. The axial hole in the lower pole shoe is preferably in the form of a central through-hole with a circular profile cross section, having a diameter that corresponds approximately to the largest profile width of the push rod. This makes it possible to guide the push rod exactly in the axial hole in the lower pole shoe, while at the same time saving the previously normal separate pressure equalizing channels, which were formed by complex eccentric holes in the lower pole shoe, since these are now formed by the free cross-sectional spaces which are produced alongside the profiled push rod in the axial hole.
A further feature for optimized-cost production of the electromagnet is that the magnet armature, which has an end face that rests on the push rod, has a central longitudinal hole with a diameter that is smaller than the largest profile width of the push rod and that is larger than the smallest profile width of the push rod. As a result, the end face of the push rod only partially covers the longitudinal hole in the magnet armature so that the longitudinal hole can be used as a pressure equalizing channel between the first chamber and the second chamber in the armature space of the electromagnet via the free cross-sectional areas of its opening. This configuration is possible only because of the separation of the magnet armature and push rod and by the profiled configuration of the push rod. It has the advantage that a pressure equalizing channel between the chambers in the armature space of the electromagnet is formed by a single, central through-hole in the magnet armature. The through-hole can be produced relatively easily and possibly even without cutting. As a result, it is possible to save the previously normal separate pressure equalizing channels, which were likewise formed by costly eccentric holes or by axial grooves in the magnet armature. Those profile sections of the profiled push rod which project beyond the opening of the longitudinal hole in the magnet armature and rest on the end face of the magnet armature ensure that, despite the shape and size of the push rod, which is guided centrally in the lower pole shoe and despite the longitudinal hole, which is likewise arranged in the central magnet armature, a contact surface which is sufficient to transmit the electromagnetically produced axial movements of the magnet armature to the push rod is provided between the magnet armature and the push rod. The required continuous contact between the magnet armature and the push rod is ensured, in a manner which allows force to be transmitted by a spring element which is operatively connected to the control piston of the hydraulic valve, and this control piston presses the push rod against the end face of the magnet armature, and produces the force equilibrium with respect to the electromagnet to which flow of current is passing.
In one refinement of the electromagnet, the push rod preferably has a polygonal profile with either rounded profile edges, or a round profile, and which is flattened on one or more sides. It is comprised of a brass alloy. Such profiles can be produced without using cutting machining operations by means of extrusion, and can likewise be cut to the appropriate length by stamping without metal cutting machining. This contributes to optimized-cost production of the electromagnet. Triangular or quadrilateral profiles are particularly suitable. In order to improve their guidance, they are rounded on their profile edges with the radius of the axial hole in the lower pole shoe, or have round profiles which have a slightly smaller diameter than the axial hole in the lower pole shoe and are designed to have one or more axial flats on their outer surface. Other suitable profiles are oval profiles or else round profiles, which are guided in an oval axial hole in the lower pole shoe, and/or the use of some other suitable material for the push rod, as well.
For use of the electromagnet according to the invention as an operating element of a hydraulic valve, which is intended for controlling an apparatus for camshaft movement, both the cross-sectional areas of the pressure equalizing channels in the lower pole shoe and those cross-sectional areas of the longitudinal hole in the magnet armature which are not covered by the end face of the push rod can each have an overall flow cross section of at least 0.5 mm2, if a normal static operating pressure of up to 10 bar is used within the hydraulic system. This overall flow cross section is considered when choosing the profile shape and the profile size for the push rod. It represents a lower limit value at which any damping effect on the magnet armature or any increase in the hysteresis due to excessively small cross sections in the pressure equalizing channels can be excluded, even at a low pressure medium temperatures and if the pressure medium viscosity is high.
Finally, for optimized-cost production of the electromagnet, the nonmagnetic metal tube in the hollow cylinder of the coil former is preferably a cup-shaped copper tube which is closed at one end, seals the coil winding against the operating fluid of the hydraulic valve, and has an inner face in the form of a guide for the magnet armature. Such a copper tube is closed at one end. It can be produced at low cost as a deep-drawn part without metal cutting machining. This makes it possible to save a pressure tube sleeve, which is also normally used in the armature space in such electromagnets and is generally comprised of a highly alloyed stainless steel. However, instead of using a cup-shaped copper tube, it is also possible to use a tube of identical construction but comprised of some other suitable material.
Furthermore, it has been found to be advantageous to provide the magnet armature and/or the inner face of the nonmagnetic metal tube with a low-friction or wear-reducing coating in order to reduce the hysteresis of the magnet armature and in order to increase the life of the electromagnet. This coating may, for example, be in the form of a PTFE coating or a tin, silver, copper, nickel or anodized coating, depending on the materials of the metal tube and of the magnet armature. In addition or else as an alternative to such a coating, it is also advantageous not to guide the magnet armature by its entire outer surface on the inner face of the magnetic metal tube, in order to further reduce the friction coefficients and the hysteresis of the magnet armature. In order to define its bearing points on the inner face of the nonmagnetic metal tube, the magnet armature is therefore machined, preferably by center-less grinding, at its upper and lower ends, with the diameter of the magnet armature between the bearing points being reduced minimally, in a known manner.
The electromagnet according to the invention, in particular a proportional magnet for operating a hydraulic valve, thus has the advantage over known electromagnets in that it is comprised of physically simple and mutually arranged individual parts whose manufacture and assembly require little effort, which considerably reduces the costs for production of the electromagnet. In particular, the separation of the magnet armature and push rod into individual parts, which are each separately guided, enables completely saving producing the eccentric pressure equalizing channels, which previously had to be additionally incorporated in the magnet armature and in the lower pole shoe and this involved considerable effort. This also completely saves the complex calibration work to avoid axis offsets between the longitudinal axes of the magnet armature, of the push rod and of the central axial hole in the lower pole shoe.